function darkObj() {
	this.a1 = 350;
	this.a2 = can.width / 2 + 150;

	this.min1 = 1;
	this.min2 = 2;
}

darkObj.prototype.draw = function() {

	var t1 = "rgba(9,48,103,0)";
	var t2 = "rgba(2,2,24,1)";
	var x = can.width / 2;
	var y = can.height / 2-250;

	if (this.a1 > this.min1) this.a1 -= 0.002*time;
	if (this.a2 > this.min2) this.a2 -= 0.002*time;


	var grd = ctx.createRadialGradient(x, y, this.a1, x, y, this.a2); //在画布上确定两个渐变的圆心和半径
	grd.addColorStop(0.001, t1); //内部渐变颜色和从哪里开始渐变
	grd.addColorStop(0.999, t2); //外部渐变颜色和渐变到哪里为止
	ctx.fillStyle = grd;
	ctx.fillRect(0, 0, can.width, can.height);

	if(this.a1 < this.min1 && this.a2 < this.min2){
		ctx.fillStyle = t2;
		ctx.arc(x,y,5,0,2* Math.PI);
		ctx.fill();
	}
}
